Most plasma processes for semiconductor circuit fabrication require the plasma reactor chamber to be maintained at a sub-atmospheric pressure using a vacuum pump coupled to the chamber. Typically, the vacuum pump is operated at a nominal constant rate, while the chamber pressure is adjusted by a butterfly valve coupled between the chamber and the vacuum pump. The butterfly valve has a rotatable disk-shaped flap whose rotational position determines the flow rate to the vacuum pump and therefore controls the chamber pressure. The valve flap typically has an O-ring around its perimeter that seats on the edge of the valve housing whenever the valve is in the closed position. The O-ring is necessary in order to ensure a seal when the valve flap is in the closed position. The O-ring suffers wear when it is in a slightly opened position at which the desired chamber pressure is achieved. Plasma and gases flowing past the O-ring react with the O-ring material and degrade it or remove it. As a result, the valve must be serviced periodically to replace the O-ring, which entails significant maintenance costs and down-time of the reactor.
Another problem is that there is a trade-off between the maximum flow capacity of the valve and its ability to regulate chamber pressure accurately. The resolution with which pressure can be controlled is roughly inversely proportional to the valve diameter. This is because control of the rotational angle of the valve flap is limited to a minimum angular excursion, depending upon the motor or servo employed to rotate the flap. The minimum angular excursion or resolution may be less than 1 degree. For a very small diameter valve flap and opening, this resolution can afford highly accurate or fine control of the chamber pressure. However, for a larger diameter valve flap or opening, movement of the flap through the minimum angular excursion causes a relatively large change in chamber pressure, so that fine control of chamber pressure is not possible. This problem can be overcome by employing a smaller diameter valve flap and opening. However, such an approach limits the rate at which the chamber can be evacuated or cleaned. For example, cleaning the chamber with NF3 gases with a fast “dump” of the cleaning gases and by-products is not possible with a small diameter valve.
What is desired is a pressure-control valve that has a very high maximum flow rate (maximum opening size) but which, despite the large maximum opening size, can control chamber pressure as accurately as a very small valve, and requires no periodic replacement of an O-ring.